There has been proposed a process utilizing electron beam irradiation to crosslink, cure or modify a coating material applied to a substrate or base, such as paint, printing ink, adhesive, pressure sensitive, etc., or other resin products, and extensive studies have been made up to the present. In this process, electrons are accelerated with a voltage applied thereto in a vacuum and the accelerated electrons are guided into a normal-pressure atmosphere, such as in the air, so that an object may be irradiated with an electron beam (EB).
Crosslinking, curing or modification by means of electron beam irradiation have the following advantages:
(1) Organic solvent need not be contained as a diluent, and thus the adverse effect on the environment is small. PA1 (2) The rate of crosslinking, curing or modification is high (productivity is high). PA1 (3) The area required for crosslinking, curing or modification is small, compared with heat drying treatment. PA1 (4) The substrate or base is not applied with heat (electron beam irradiation is applicable to those materials which are easily affected by heat). PA1 (5) Post-treatment can be immediately carried out (cooling, aging, etc. are unnecessary). PA1 (6) It is necessary that the conditions for electrical operation be controlled, but the required control is easier than the temperature control for heat drying treatment. PA1 (7) Neither initiator nor sensitizing agent is required, and thus the final product contains less impurities (quality is improved).
According to conventional electron beam irradiation techniques, however, a high-energy electron beam is used to crosslink, cure or modify objects at a high rate, and no consideration is given to energy efficiency.
Conventional techniques are also associated with problems such as the problem that much initial investment is required because of large-sized apparatus, the problem that inerting by means of an inert gas such as nitrogen, which is high in running cost, is needed in order to eliminate inhibition to the reaction at surface caused due to generation of oxygen radical, and the problem that shielding from secondary X-ray is required.
Specifically, conventional electron beam curing or crosslinking uses an acceleration voltage which is usually as high as 200 kV to 1 MV and thus x-rays are generated, making it necessary to provide a large-scale shield for the apparatus. Also, where such a high-energy electron beam is used, care must be given to possible adverse influence on the working environment due to generation of ozone. Since the reaction at the surface of an object is inhibited due to generation of oxygen radical, moreover, inerting by means of an inert gas such as nitrogen is required.
Further, an electron beam generated with a high acceleration voltage applied thereto penetrates to a great depth and thus can sometimes deteriorate the substrate or base such as a resin film or paper. In the case of paper, for example, disintegration of cellulose due to the breakage of glycoside bond takes place at a relatively small dose, and it is known that deterioration in the folding strength is noticeable even at an irradiation dose of 1 Mrad or less. Especially in the case where the substrate or base has a coating material (printing ink, paint, adhesive, etc.) of 0.01 to 30 .mu.m thick printed thereon or applied thereto, the thickness of the coating material is small and the substrate or base may have an exposed surface having no coating material thereon, often giving rise to a problem that the substrate or base is deteriorated.
Accordingly, there is a demand for low-energy electron beam irradiation apparatus and process which use low acceleration voltage and which permit reduction in size of the apparatus.
To meet the demand, various apparatus and process using low acceleration voltage for electron beam irradiation have been proposed, and Japanese Patent Disclosure (KOKAI) No.5-77862, for example, discloses a process for 30-Mrad irradiation at 200 kV, as an example of electron beam irradiation at a low acceleration voltage. However, even with this process, the acceleration voltage is not low enough to prevent deterioration of the substrate or base and also inerting is required.
Japanese Patent Disclosure No. 6-317700 discloses an apparatus and process for irradiating an electron beam with the acceleration voltage adjusted to 90 to 150 kV. According to this technique, a titanium or aluminum foil of 10 to 30 .mu.m in thickness is used as a window material which intervenes between an electron beam generating section of the electron beam irradiation apparatus, in which electrons released from the cathode are guided and accelerated to obtain an electron beam, and an irradiation room in which an object is irradiated with the electron beam.
However, even with this technique, when the acceleration voltage is set to 100 kV or less in actuality, the penetrating power of the electron beam is very low, and since most of the electron beam is absorbed by the window material, the electron beam cannot be efficiently guided into the irradiation room. Also, the temperature of the window material may possibly rise up to its heat resistance temperature or higher. Consequently, the apparatus is in practice used with the acceleration voltage set at a level higher than 100 kV, and even with such acceleration voltage, deterioration of the substrate or base can be caused.
Thus, the electron beam curing technique has been attracting attention as a process which serves to save energy, does not require the use of solvent and is less harmful to the environment, but it cannot be said that the technique has been put to fully practical use because of the aforementioned problems.